This application is based upon and claims the benefit of priority of Japanese Patent Application No. H.11-307490 filed on Oct. 28, 1999, the content of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a method of manufacturing a spark plug for internal combustion engine, in particular, a method of bonding a noble metal chip to a center or ground electrode by welding.
2. Description of Related Art
It is well known to bond a noble metal chip to a center or ground electrode only by laser welding, as shown in JP-A-6-45050. However, in a case of bonding only by laser welding, the noble metal chip has to be held by a holding jig or tool when the chip is bonded by laser welding. Accordingly, a construction of a laser welding equipment becomes complicated.
Further, as disclosed in JP-No.2921525, it is known to fix provisionally at first the noble metal chip to the center or ground electrode by resistance welding and to bond finally the same by laser welding.
However, when the noble metal chips are bonded to the respective center or ground electrodes provisionally by a conventional resistance welding method, in which a current amount to be supplied and a time period for current supply are controlled at preset values, and finally by a conventional laser welding method, bonding strength of the noble metal chips to the respective center or ground electrodes is likely to fluctuate.
As a result of an extensive research and experimental test, it is contemplated that this bonding strength fluctuation is caused by a fact that structure of molten portions to be formed by laser welding is uneven. Further, the uneven molten portion structure is affected by fluctuation of respective lengths of the noble metal chips to be embedded into the center or ground electrodes when the provisional resistance welding is conducted.
The experimental test result further reveals that, when the resistance welding on the noble metal chips is implemented under conditions that the current amount to be supplied and the time period for current supply are constant, the embedding length of the noble metal chips into the center or ground electrodes fluctuates because of, for example, uneven surface roughness of cutting surfaces of the noble metal chips or uneven surface roughness of surfaces of the center or ground electrodes, on which the noble metal chips are placed, respectively.
When the noble metal chip, for example, including Ir as a main composition and having a high melting point, is fixed by resistance welding, surface roughness of the surface on which the noble metal chip and the center or ground electrode are in contact with each other largely affects heat energy to be generated on a boundary surface between the noble metal chip and the center or ground electrode. Therefore, the resistance welding at the constant current amount and the constant time period is not sufficient enough to secure a stable and accurate embedding length of the noble metal chip into the center or ground electrode.
The present invention has been made in view of the above mentioned problem, and an object of the present invention is to provide a method of manufacturing spark plug in which a welding condition of provisional resistance welding is controlled or adjusted to secure a uniform predetermined final embedding amount of the noble metal chip before the final laser welding is implemented. As a result, stable and accurate bonding strength of the noble metal chip to a center or ground electrode can be assured, since the construction of molten portion formed by laser welding is uniform and stable.
To achieve the above object, the steps are comprised of, at first, putting a noble metal chip on a leading end of one of the center and ground electrodes so that a surface of the noble metal chip may come in contact with a surface of the leading end of the one of the center and ground electrodes, next, executing resistance welding provisionally in such a manner that current is passed through the noble metal chip and the leading end of the one of the center and ground electrodes, while the noble metal chip is pressed toward the leading end of the one of the center and ground electrodes, so as to fix the noble metal chip to the one of the center and ground electrodes in a state that a part of the noble metal chip is embedded into the one of the center and ground electrodes, and, then, executing laser welding finally so as to melt a circumference of a portion where the noble metal chip is embedded into the one of the center and ground electrodes.
When the provisional welding is executed, at least one of a current supply amount and a current supply time period is controlled according to at least one of a transit embedding length and a transit embedding speed of the noble metal chip to the one of the center and ground electrodes to establish a predetermined final embedding amount of the noble metal chip to the one of the center and ground electrodes.
It is preferable that first and second electrodes (upper and lower electrodes) of a resistance welding equipment are set to be electrically conductive with the noble metal chip and the one of the center and ground electrodes, while the first electrode presses the noble metal chip toward the one of the center and ground electrodes. The current supplied between the first and second electrodes by a power source of the resistance welding is controlled by a transit moving length or a transit moving speed of at one of the first and second electrodes, which corresponds to the transit embedding length or the transit embedding speed of the noble metal chip to the one of the center and ground electrodes.
Furthermore, the predetermined final embedding amount of the noble metal chip is, preferably, not larger than 0.1 mm to obtain an adequate alloy ratio of the noble metal chip to the one of the center and ground electrodes.